1. Field of the Invention
The present invention relates to projection display devices, and more particularly, to a projection display device which projects a still image or a moving image, which is formed on a screen such as a computer screen or a TV screen, onto a projection plane such as a screen or a wall by scanning light beams.
2. Description of the Related Art
Hitherto, so-called projection laser displays which scan and project laser beams of three colors, that is, red, green, and blue, have been widely available, and an exemplary projection laser display is described in Electronics and Communications in Japan, Vol. 68, No. 4, pp. 387-394. In this example, red, green, and blue laser beams are amplitude modulated by an optical modulator and are combined on a single optical axis, and then an image is projected onto a screen by scanning the combined beams two-dimensionally with horizontal and vertical optical scanning elements. Although known optical scanning elements generally include an acousto-optic deflector, an electro-optic deflector, and a mechanical deflector, the acousto-optic and the electro-optic deflectors are impractical due to problems of a small deviation angle and a color dispersion; accordingly, the mechanical deflector is generally used. Exemplary mechanical deflectors include a polygonal mirror and a galvanometer mirror. Tone and hue of the image are adjusted by modulating the intensity of light emitted from the laser beam sources.
FIG. 10 illustrates the configuration of a video projector disclosed in U.S. Pat. No. 5,485,225 in which laser beams having different wavelengths are overlaid on a single light path by dichroic mirrors 118 and 119 and then the overlaid beams are projected by a beam scanner 130. FIG. 10 also illustrates 1) a first component group 101 having light sources 104-106, light modulators 107-109, light bundles 114-116 and an optical arrangement 117 with mirrors 118-121, 2) a second component group 102 with a deflecting device 130 having a polygonal mirror 131 and a swiveling mirror 132, an optical system 134 and a screen 136, and 3) a third component group 103 including common light bundle 122, first optical system 124, optical fiber 126 and second optical system 128. The luminous intensity of the radiated laser beam is controlled by modulating the beam with an external light modulator or by directly modulating the beam from a light emitting diode.
FIG. 11 illustrates the configuration of an image projector disclosed in Japanese Patent Laid-Open No. 11-305710 in which, instead of scanning light beams, the emission time of laser pulses for the red, green, and blue colors are shifted from each other in order to commonly use a single light valve 206 for the pulses of light of the three colors. Although the projection time of the laser pulses on a projection plate 208 shifts from color to color, the human eye observes the projected image as a color image because of the visual integration effect of the human eye. The image projector is also shown to include laser light sources 201-203, dichroic prism 204, integrator 205 and projections 207. Since pulsed lasers are used as the red, green, and blue light sources, light modulation is performed by modulating the intensity of light with the light valve, which is disposed after the light sources.
However, known projection display devices using lasers, including the above projectors, have the following problems.
When light intensity modulation is performed while scanning the laser beams, the laser beams are spatially coupled with each other, leading to poor separation between adjacent projected pixels, thereby causing color drift, uneven brightness, uneven tone, and so forth; those problems are especially acute in the case of high-resolution images.
Also, a color combining component is needed for combining a plurality of laser beams on a single light path, wherein the laser beams for color display have different oscillating wavelengths from each other, thereby resulting in a projection display device with a large size, a complicated parts-mounting process, and high cost.
Furthermore, when pulsed lasers are used, since it is difficult to restrain intensity variations from one pulsing shot to another by feedback control, there is a risk of uneven image display problems.
Moreover, oscillating red, green, and blue light pulses in a time sequential manner causes the pixel clock for these three colors to extend, resulting in a smaller number of displayable image resolution points. To avoid this problem, modulation of each color must be performed in one third of the pixel clock width.